Former deputy party chairman Lord Ashcroft warned that the man known as the "Wizard of Oz" for his multiple successes in Australian elections was a "recipe for... conflict and confusion".

But Mr Johnson, whose two successful London mayoral campaigns were run by Mr Crosby, has played down his right-wing reputation and urged the Prime Minister to "break the piggy bank" to get him on board.

The long-anticipated appointment was revealed days after the Tories suffered a heavy reverse in the Corby by-election - losing a mid-term seat to Labour for the first time since 1997 on a swing of nearly 13%.

Elections for police commissioners were also hit by record-low turnouts on Thursday in a humiliating blow to a flagship law and order policy championed by Mr Cameron.

Opinion polls tonight gave Labour a 12-point and 11-point lead just over halfway through the five-year term of the power-sharing administration with the Liberal Democrats.

Many Conservatives have expressed concerns about a perceived lack of grip and focus in Number 10.

Tim Montgomerie, editor of influential website ConservativeHome, said Mr Crosby had "a good record of success and is perhaps the Alpha Male that Team Cameron has been missing since Andy Coulson exited.

"Coming on top of the decision to target 20 Lib Dem MPs, it is a sign that Team Cameron is re-gearing for the kind of ambitious campaign that will be necessary to win a majority at the next election," he wrote.

The appointment was immediately surrounded in controversy however, with Mr Johnson forced to defend the strategist against claims about a foul-mouthed rant during this years London mayoral election.

The Mail on Sunday quoted a source suggesting he had told Mr Johnson to concentrate on traditional Tory voters instead of 'f****** Muslims'.

A spokesman for Mr Crosby told the newspaper that he had "absolutely no recollection" of using the phrase and said it was "mischievous" to suggest he was racist "in any way".

"I don't recognise this characterisation. He's the best campaign manager I've ever seen. The Conservative Party has done well to retain his services."

The mayor's official spokesman said Mr Johnson had "no recollection of the alleged conversation in question.

"The Mayor would recognise that there were debates over the direction of strategy during the campaign. What he would not recognise is any suggestion of abusive or offensive language directed at any particular group of voters from any member of his campaign staff."